High-dose L-theanine-caffeine combination improves neurobehavioural and neurophysiological measures of selective attention in acutely sleep-deprived young adults: a double-blind, placebo-controlled crossover study.

L-theanine, an amino acid found in tea; and caffeine, found in tea and coffee, are claimed to enhance attention. We conducted a double-blind, placebo-controlled, counterbalanced, two-way crossover trial to determine the acute effects of a high-dose L-theanine-caffeine combination on neurobehavioural (reaction time) and neurophysiological [P3b cognitive event-related potential (ERP)] measures of selective attention in acutely sleep-deprived healthy adults. Thirty-seven overnight sleep-deprived healthy adults (aged 22-30 years, 21 men) completed a computerised traffic-scene-related visual stimulus discrimination task before and 50 minutes after ingesting 200 mg L-theanine-160 mg caffeine combination, or a placebo. The task involved selectively responding to imminent accident scenes (20% probability) while ignoring randomly intermixed, more frequent safe scenes (80% probability). A 32-channel EEG was recorded concurrently to derive ERPs. The L-theanine-caffeine combination significantly improved the hit rate (P = 0.02) and target-distractor discriminability (P = 0.047), compared to the placebo. Although both L-theanine-caffeine combination (△ = 52.08 ms, P < 0.0001) and placebo (△ = 13.97 ms, P = 0.024) improved reaction time to accident scenes, the pre-post-dose reaction time improvement of the L-theanine-caffeine combination was significantly greater than that of placebo (△ = 38.1 ms, P = 0.003). Compared to the placebo, the L-theanine-caffeine combination significantly increased the amplitudes and reduced the latencies of P3b ERP component. Our findings suggest that L-theanine-caffeine combination improves the accuracy and speed of deploying selective attention to traffic scenarios in sleep-deprived individuals. This improvement is brought about by greater and faster neural resource allocation in the attentional networks of the brain.